(1) Field of the Invention
The present invention relates to an optical transmission system and an optical transmission method, for transmitting a wavelength division multiplexed light containing a plurality of optical signals of different wavelengths, by the use of an optical fiber, and in particular, relates to technology for adding optical signals in a coarse wavelength division multiplexing system.
(2) Related Art
A dense wavelength division multiplexing (hereafter referred to as DWDM) system, which deals with a plurality of optical signals of 0.8 nm inter-wavelength and further at 0.4 nm, has been developed, and is now in practical use, as a communication system enabling a dramatic increase in transmission capacity. This DWDM system can normally be operated with a number of signals of between 32 and 128 waves, however, in order to accurately manage wavelengths, requires expensive light source and wavelength management equipment. Therefore, the cost of the DWDM system is high even when operated with a small number of signals.
Therefore, a system is earnestly desired, which realizes a low-cost WDM system, which is operated with a small number of signals by increasing a signal wavelength interval to an extent wherein a highly accurate wavelength management becomes unnecessary. Recently, a coarse wavelength division multiplexing (hereafter referred to as CWDM) system capable of dealing with up to optical signals of 8 waves (8 channels) having 20 nm inter-wavelength, has been developed and is now in commercial use. A demand for this CWDM system extends over a wide range of from an application in access having a comparatively small transmission capacity (for example, metropolitan areas and the like) to LANs within buildings. Furthermore, owners of optical fiber networks such as public organizations, electric power companies, and communication carrier manufacturers and the like have commenced the line renting using a dark fiber (optical fiber installed but not in use). In this case, as well, when the transmission capacity is comparatively small, since it is necessary to provide a low-cost service, and the aforementioned CWDM system is suitable.
However, it is demanded that the CWDM system is capable to hold not only optical signals in conformity with Synchronous Digital Hierarchy (SDH) being the international standard for a high-speed digital communication system using an optical fiber, but also optical signals used in an Ethernet data communication. In order to respond to this demand, a technology for enabling the number of signals to be added up to 8 or more channels in the existing CWDM system is necessary.
Conventionally, a system for adding the number of signals in the CWDM system so as to exceed a general standard value, has not been supported by the equipment manufacturer, and a maximum of 8 channels (16 channels at full-band) could be dealt with. Consideration has recently been given to add the number of signals by merging CWDM and DWDM systems (refer to the following first and second literatures).
First Literature
ADVA Optical Networking, “ADVA Launches Major CWDM Feature Release for FSP 2000”, [online], Oct. 28, 2003, Internet <URL: http://www.advaoptical.com/adva_press.asp?id=66&action=view&msgid=331>
Second Literature
H. Hinderthür and L Friedric, “WDM hybrid transmission based on CWDM plus DWDM”, Lightwave Europe, volume 2, Issue 7, [online], July 2003, Internet <URL: http://Iw.pennnet.com/Articles/Article=_Display.cfm?Section=ARCHI& ARTICLE_ID=183295&VERSION_NUM=1&p=63>
However, in the above described conventional technology, only the method of connecting optical signals for the DWDM system to the CWDM system has been considered, and the method of transmitting the connected optical signals for the DWDM system over the optical fiber has not been considered. Therefore, an effective optical fiber transmission has been difficult. That is to say, in the case where optical signals of a plurality of wavelengths operated in the DWDM system are connected directly to the CWDM system while the setting of the power, the inter-wavelength or the like thereof being unchanged, and the number of signals in the CWDM system are added, such a problem is considered in that the waveform deterioration due to a nonlinear effect of the optical fiber or an increase of noise and the like due to cross-talk caused by a characteristic of an optical multiplexer/demultiplexer for the CWDM system occurs in the added optical signal of the DWDM side, resulting in the reduction in transmission quality.